Apparatus and methods for determining the minority-carrier diffusion length (L) using the surface photovoltage (SPV) method are well known. In brief, the principle of the diffusion-length (L) determination requires the illumination of a specimen surface with monochromatic radiation of energy slightly greater than the bandgap of the semiconductor. Electron-hole pairs are produced and diffuse to the illuminated surface where they are separated by the electric field of the depletion region (i.e., the surface-space-charge region) to produce a surface photovoltage. A portion of the surface photovoltage signal is coupled to an amplifier for amplification and measurement. The photon flux (photons per sq. cm. per second) is adjusted to produce the same magnitude of surface photovoltage at various wavelengths of illumination. The photon flux required to produce this constant magnitude surface photovoltage signal for each wavelength is conveniently plotted on the ordinate against the reciprocal of the absorption coefficient on the abscissa. The resultant plot is typically linear and is extrapolated to the zero photon flux intercept on the negative abscissa. The magnitude of this intercept value is the effective diffusion length (L). For a more detailed description of the theory and background for this method, see an article "A Method For The Measurement Of Short Minority Carrier Diffusion Lengths In Semiconductors", by A. M. Goodman in the Journal of Applied Physics, Vol. 32, No. 23, pp. 2550-2552, December 1961, and the article by A. M. Goodman entitled "Improvements In Method And Apparatus For Determining Minority Carrier Diffusion Length", International Electron Devices Meeting, December 1980, pp. 231-234. The American Society for Testing and Materials has adopted a standard using this method which is published as ASTM F 391-78. The ASTM standard, when implemented according to the block diagram of FIG. 1 of ASTM F 391-78, is provided particularly for determining the diffusion length (L) for minority carriers in silicon but the method in general may be used for other semiconductor materials.
See U.S. Pat. No. 4,333,051, incorporated herein by reference thereto, entitled "Method And Apparatus For Determining Minority Carrier Diffusion Length In Semiconductors", issued on June 1, 1982 to A. M. Goodman for a description of an apparatus using this principle in which a servo system maintains a constant predetermined value of the surface photovoltage thereby allowing the measurements to be carried out in a relatively short time. The surface photovoltage pickup electrode described in this patent tends to reduce the effects of drift caused by laterally diffusing minority carriers during a test.
As described in the above-identified U.S. Pat. No. 4,333,051, the SPV method for determining diffusion length in semiconductor materials can be used to reveal contamination, particularly by heavy metals, that may have occurred during one or more steps of the processing of semiconductor materials, typically in wafer form. A specimen wafer to serve as a monitor is placed together with the wafers to be processed in a furnace or other environments for processing. Any contamination of the wafers during the processing steps, particularly the furnace steps, will also contaminate the specimen material. The contamination manifested as a decrease in the diffusion length of that specimen is an indication of such contamination.
However, after growing epitaxial layers on semiconductor wafers, it has been difficult or impossible to determine the diffusion length of such a layer because its thickness is typically less than three times the diffusion length of the material. Unless such an epitaxial layer is strongly contaminated by heavy metal impurities, the actual diffusion length of the layer will be significantly larger than the one-third thickness of the epitaxial layer. W. E. Phillips describes in an article entitled "Interpretation Of Steady-State Surface Photovoltage Measurements In Epitaxial Semiconductor Layers", Sol. St. Elec. 15, 1097 (1972), a detailed analysis for extracting the actual diffusion length in a thin epitaxial layer from the apparent or effective value of the diffusion length. However, in the most common range of interest where the epitaxial layer thickness is less than or about the same as the apparent diffusion length, the extracted value of the diffusion length in the epitaxial layer, according to Phillips' technique, is a very sensitive function of the system parameters and is subject to large errors. The parameters are often not known to the accuracy needed to obtain usefully accurate values of the diffusion length in the epitaxial layer.